Tm Calculator

The Tm calculator estimates the melting temperature of a DNA primer or oligonucleotide directly from its base sequence. The melting temperature, written as Tm, is the temperature at which half of the double-stranded molecules in a sample have separated into single strands. It is one of the most important numbers in molecular biology because it controls how primers bind their templates during the annealing step of PCR, qPCR, sequencing, site-directed mutagenesis, and hybridization assays.

This melting temperature calculator accepts a sequence written 5' to 3' and ignores any character that is not A, T, G, or C, so you can paste a primer with spaces or formatting without cleaning it first. As soon as the cleaned sequence reaches at least eight bases, the tool reports the predicted Tm, a suggested annealing temperature (Tm minus 5 °C), the sequence length, the GC content, the approximate molecular weight in daltons, and the count of each base. These outputs let you judge primer quality at a glance instead of running a separate GC content calculator or molecular weight calculator.

Because no single equation is correct for every length and salt condition, the calculator offers five methods. A short primer behaves differently from a long oligo, and a reaction run in high salt melts at a higher temperature than the same primer in dilute buffer. Choosing the right method, and entering realistic salt, magnesium, and DMSO values, is what separates a rough guess from a usable primer design number.

The Tm calculator implements five established approaches. Each is suited to a different sequence length or accuracy requirement, and the calculator applies the same salt, magnesium, and DMSO corrections on top of whichever method you select.

The Basic and Wallace rules add 2 °C for every A or T and 4 °C for every G or C; they are fast but ignore length, salt, and stacking, so they overestimate Tm for anything longer than a short oligo. The GC content (Marmur-Doty) method scales the contribution of the combined G and C count by the sequence length N, which makes it more reliable for medium primers. The salt-adjusted equation folds the sodium concentration and length directly into the result, so it responds to buffer changes. The nearest neighbor method is the most accurate and is described in detail below.

The default and most accurate option is the nearest-neighbor model. Instead of treating bases independently, it recognises that the stability of a duplex depends on which base sits next to which. The calculator walks through every overlapping pair of bases (dinucleotide) in your primer and adds up the enthalpy (ΔH°) and entropy (ΔS°) contributions from the SantaLucia 1998 unified parameter set, measured in cal/mol and cal/mol·K respectively.

After summing all 16 possible dinucleotide steps, the tool adds a small initiation correction (+100 cal/mol to ΔH° and −2.8 cal/mol·K to ΔS°) to account for helix formation, then solves the thermodynamic relationship for Tm and converts from Kelvin to Celsius. Finally it applies the monovalent-salt term using the sodium concentration you entered. This is why two primers with identical GC content can still report different melting temperatures: their dinucleotide stacks differ.

The nearest-neighbor approach is the standard used by professional primer-design pipelines, so when you need a number you can act on, leave the method on Nearest Neighbor (most accurate) and enter your true primer concentration and salt.

Melting temperature is not a property of the sequence alone — the surrounding chemistry matters. After the chosen method produces a base value, this Tm calculator applies three buffer corrections so the result reflects your real reaction.

Sodium (salt) correction. Monovalent cations such as Na+ shield the negatively charged phosphate backbone and stabilise the duplex, raising Tm. In the salt-adjusted and nearest-neighbor methods the tool adds 16.6 × log10 of the sodium concentration expressed in molar. Because 50 mM equals 0.05 M and the logarithm of a number below 1 is negative, modest salt concentrations actually pull the corrected Tm downward relative to a 1 M reference, which is why entering a realistic 50 mM Na+ noticeably lowers the displayed value.

Magnesium correction. Mg2+ ions are essential cofactors for polymerases and strongly affect duplex stability. When magnesium is greater than zero, the calculator applies a term of 0.7 × log10([Mg2+]/1000) × 10. At the default 1.5 mM this term is large and negative (about −19.8 °C), so the predicted Tm drops considerably. If you want to see the uncorrected melting temperature, simply set Mg2+ to 0 and the magnesium term is skipped entirely.

DMSO correction. Dimethyl sulfoxide is a common PCR additive that disrupts base pairing, so each percent of DMSO lowers Tm. The calculator subtracts 0.6 °C for every 1% DMSO you enter, matching the widely cited 0.5–0.6 °C per percent estimate.

Enter your oligonucleotide in the sequence box, written 5' to 3'. The Tm calculator strips out anything that is not A, T, G, or C, so pasted FASTA fragments or spaced-out sequences are handled automatically. Remember that the cleaned sequence must contain at least eight bases before any result appears.

1. Sequence: paste or type the primer; only the four standard DNA bases are counted.
2. Calculation method: choose Nearest Neighbor for accuracy, or a simpler rule for a quick estimate.
3. Primer concentration (nM): the total strand concentration; used by the nearest-neighbor equation.
4. Salt concentration (mM Na+): your monovalent cation level, typically 50 mM for standard PCR.
5. Mg2+ concentration (mM): magnesium in the reaction; set to 0 to disable the magnesium correction.
6. DMSO (%): any DMSO additive, which lowers the predicted Tm.

The result panel shows the melting temperature, the suggested annealing temperature (Tm minus 5 °C), the sequence length in base pairs, the GC content percentage, the approximate molecular weight, and a breakdown of how many A, T, G, and C bases the primer contains. Read these alongside the built-in PCR guidelines to judge whether the primer is well designed.

A good PCR primer usually has a melting temperature between 55 and 65 °C, a length of 18 to 25 nucleotides, and a GC content of 40 to 60%. The Tm calculator surfaces all three numbers so you can iterate quickly. The single most useful output for benchwork is the annealing temperature: starting roughly 5 °C below the lower primer Tm gives the polymerase a good chance of specific binding without sacrificing yield.

When you design a primer pair, calculate the Tm of both primers and keep them within about 5 °C of each other. A large mismatch means one primer binds efficiently while the other lags, producing weak or off-target product. The reported GC content also flags problems early: too low and the duplex is unstable, too high and you risk secondary structure and difficult denaturation.

Finally, favour a 3' terminal G or C — the so-called GC clamp — to anchor the priming end, and avoid runs of four or more identical bases that can cause slippage. Combined with the molecular weight and base-composition readouts, this melting temperature calculator gives you everything needed to screen candidate primers before ordering them, saving both time and reagent cost at the bench.